Treg-Centric View of Immunosuppressive Drugs in Transplantation: A Balancing Act.

Regulatory CD4+ Foxp3+ T cells (Tregs) are critical in controlling immunity and tolerance. Thus, preserving Treg numbers and function in transplanted patients is essential for the successful minimization of maintenance immunosuppression. Multiple cellular signals control the development, differentiation, and function of Tregs. Many of these signals are shared with conventional Foxp3- T cells (Tconv) and are targeted by immunosuppressive drugs, negatively affecting both Tregs and Tconv. Because intracellular signals vary in optimal intensity in different T cell subsets, improved specificity in immunosuppressive regimens must occur to benefit long-term transplant outcomes. In this regard, recent advances are gradually uncovering differences in the signals required in Tregs and Tconv biology, opening the door to new potential therapeutic approaches to either enhance or spare Tregs. In this review, we will explain the prominent cell signaling pathways critical for Treg maintenance and function, while reporting the effects of immunosuppressive drugs targeting these signaling pathways in clinical transplantation settings.

Targeting the Notch Pathway to Prevent Rejection.

Immune rejection is mediated by a complex interplay of cellular and humoral mechanisms. Current therapeutic strategies, which rely on global immunosuppression, can result in serious complications and are not completely effective. Notch signaling is a cell-to-cell communication pathway that plays an important role during T cell development and in the regulation of peripheral immune responses. Initial work, performed mainly through gain-of-function approaches, paradoxically identified Notch as an inducer of tolerance; however, recent studies using loss-of-function approaches in mouse models of transplant rejection and graft-versus-host disease have clarified an important role for Notch as a central mediator of T cell alloreactivity. Short-term inhibition of individual Notch ligands in the peritransplant period had long-lasting protective effects. In a vascularized heart allograft model, blockade of Delta-like Notch ligands dampened both cellular and humoral rejection. In this minireview, we summarize current knowledge about the role of Notch signaling during allograft rejection and provide an overarching mechanism through which Notch acts to promote T cell pathogenicity and allograft damage. We propose that targeting elements of the Notch pathway could provide a new therapeutic approach to prevent allograft rejection.

Inflammation Causes Resistance to Anti-CD20-Mediated B Cell Depletion.

B cells play a central role in antibody-mediated rejection and certain autoimmune diseases. However, B cell-targeted therapy such as anti-CD20 B cell-depleting antibody (aCD20) has yielded mixed results in improving outcomes. In this study, we investigated whether an accelerated B cell reconstitution leading to aCD20 depletion resistance could account for these discrepancies. Using a transplantation model, we found that antigen-independent inflammation, likely through toll-like receptor (TLR) signaling, was sufficient to mitigate B cell depletion. Secondary lymphoid organs had a quicker recovery of B cells when compared to peripheral blood. Inflammation altered the pharmacokinetics (PK) and pharmacodynamics (PD) of aCD20 therapy by shortening drug half-life and accelerating the reconstitution of the peripheral B cell pool by bone marrow-derived B cell precursors. IVIG (intravenous immunoglobulin) coadministration also shortened aCD20 drug half-life and led to accelerated B cell recovery. Repeated aCD20 dosing restored B cell depletion and delayed allograft rejection, especially B cell-dependent, antibody-independent allograft rejection. These data demonstrate the importance of further clinical studies of the PK/PD of monoclonal antibody treatment in inflammatory conditions. The data also highlight the disconnect between B cell depletion on peripheral blood compared to secondary lymphoid organs, the deleterious effect of IVIG when given with aCD20 and the relevance of redosing of aCD20 for effective B cell depletion in alloimmunity.

Codominant Role of Interferon-γ- and Interleukin-17-Producing T Cells During Rejection in Full Facial Transplant Recipients.

Facial transplantation is a life-changing procedure for patients with severe composite facial defects. However, skin is the most immunogenic of all transplants, and better understanding of the immunological processes after facial transplantation is of paramount importance. Here, we describe six patients who underwent full facial transplantation at our institution, with a mean follow-up of 2.7 years. Seum, peripheral blood mononuclear cells, and skin biopsy specimens were collected prospectively, and a detailed characterization of their immune response (51 time points) was performed, defining 47 immune cell subsets, 24 serum cytokines, anti-HLA antibodies, and donor alloreactivity on each sample, producing 4269 data points. In a nonrejecting state, patients had a predominant T helper 2 cell phenotype in the blood. All patients developed at least one episode of acute cellular rejection, which was characterized by increases in interferon-γ/interleukin-17-producing cells in peripheral blood and in the allograft's skin. Serum monocyte chemotactic protein-1 level was significantly increased during rejection compared with prerejection time points. None of the patients developed de novo donor-specific antibodies, despite a fourfold expansion in T follicular helper cells at 1 year posttransplantation. In sum, facial transplantation is frequently complicated by a codominant interferon-γ/interleukin-17-mediated acute cellular rejection process. Despite that, medium-term outcomes are promising with no evidence of de novo donor-specific antibody development.

Risk of metabolic complications in kidney transplantation after conversion to mTOR inhibitor: a systematic review and meta-analysis.

Mammalian target of rapamycin (mTOR) inhibitors have been used in transplantation with the hope of minimizing calcineurin inhibitor (CNI)-induced nephrotoxicity. However, mTOR inhibitors are also associated with a range of side effects, including metabolic complications. We aimed to determine the risks of metabolic complications after the conversion from CNI to mTOR inhibitor postkidney transplant. A systematic search in PubMed up to September 2013 identified nine relevant trials (a total of 2323 patients). The primary end points were the relative risks (RRs) of new-onset diabetes after transplant (NODAT) and hypercholesterolemia. The overall RRs of NODAT and hypercholesterolemia associated with mTOR inhibitors were 1.32 (95% confidence interval [CI] 0.92-1.87) and 2.15 (95% CI 1.35-3.41), respectively, compared with CNI-based regimen. Subgroup analyses revealed no differences in the incidence of NODAT or hypercholesterolemia between sirolimus- versus everolimus-based regimen, or between early versus late conversion. Analyses of secondary outcomes revealed a higher risk of acute rejection, proteinuria and anemia, but no difference in the risk of opportunistic infections after mTOR inhibitor conversion. In conclusion, the conversion from CNI to mTOR inhibitor in low-to-moderate risk kidney transplant recipients was associated with nonsignificant trend toward increased risk of NODAT and significant increase in hypercholesterolemia, acute rejection, proteinuria and anemia.

Dyslipidemia and its therapeutic challenges in renal transplantation.

Cardiovascular disease is the leading cause of mortality in kidney transplant recipients. Dyslipidemia is a common finding after renal transplantation and a significant risk factor in the development of coronary heart disease. Although a causal relationship with cardiovascular mortality has not been proven in the transplant population, it is reasonable to extrapolate data from the general population and aggressively treat posttransplant dyslipidemia. Statins are considered the agents of choice, though their use may be complicated by drug misadventures. Pravastatin, fluvastatin and pitavastatin are considered to be the safest statins to use in this population; however, given their low-potency, a high-potency statin, such as atorvastatin, may be necessary in patients with significant dyslipidemia. In this article, we discuss the etiology of and treatment strategies for dyslipidemia in renal transplant recipients based on a literature review of potential therapeutic adverse effects and benefits in this population. We will also evaluate the reasons for and consequences of the latest Food and Drug Administration (FDA) warnings regarding the use of simvastatin.

Role of the PD-1 pathway in the immune response.

Understanding immunoregulatory mechanisms is essential for the development of novel interventions to improve long-term allograft survival. Programmed death 1 (PD-1) and its ligands, PD-L1 and PD-L2, have emerged as critical inhibitory signaling pathways that regulate T cell response and maintain peripheral tolerance. PD-1 signaling inhibits alloreactive T cell activation, and can promote induced regulatory T cell development. Furthermore, the upregulation of PD-L1 on nonhematopoietic cells of the allograft may actively participate in the inhibition of immune responses and provide tissue-specific protection. In murine transplant models, this pathway has been shown to be critical for the induction and maintenance of graft tolerance. In this review, we discuss the current knowledge of the immunoregulatory functions of PD-1 and its ligands and their therapeutic potential in transplantation.

Deleterious effect of CTLA4-Ig on a Treg-dependent transplant model.

Blockade of the B7:CD28 costimulatory pathway has emerged as a promising therapy to prevent allograft rejection. However, results from the belatacept phase III clinical trial demonstrated a higher rejection rate when compared to cyclosporine, raising concern about potential deleterious effects of this agent. In this study, we investigated the consequences of B7:CD28 blockade by hCTLA4Ig on regulator T cell (Treg) generation in different major histocompatibility complex (MHC) mismatch transplant models. Administration of hCTLA4Ig significantly decreased the amount of Tregs in B6 WT animals and this effect was predominant in thymus-induced Tregs (Helios(+) ). Although hCTLA4Ig prevented rejection in a fully allogeneic mismatch model, it accelerated rejection in a MHC class-II mismatch model (MST = 26, p < 0.0001), in which long-term allograft survival is dependent on Tregs. This accelerated rejection was associated with a marked reduction in thymus-induced Tregs and led to a higher effector/regulatory T-cell ratio in secondary lymphoid organs and in the allograft. This study confirms the importance of the B7:CD28 pathway in Treg homeostasis in an in vivo transplant model and suggests that hCTLA4Ig therapy may be deleterious in circumstances where engraftment is dependent on Tregs.

Essential role of PDL1 expression on nonhematopoietic donor cells in acquired tolerance to vascularized cardiac allografts.

The PD1:PDL1 pathway is an essential negative costimulatory pathway that plays a key role in regulating the alloimune response. PDL1 is expressed not only on antigen-presenting cells (APCs) but also cardiac endothelium. In this study, we investigated the importance of PDL1 expression on donor cardiac allograft in acquired transplantation tolerance in a fully MHC-mismatched model. We generated PDL1 chimeric mice on B6 background that expressed PDL1 on either hematopoietic cells or nonhematopoietic cells of the heart. Sham animals were used as controls. These hearts were then transplanted into BALB/c recipients and treated with CTLA4-Ig to induce tolerance. Cardiac endothelium showed significant expression of PDL1, which was upregulated upon transplantation. While the absence of PDL1 on hematopoietic cells of the heart resulted in delayed rejection and prevented long-term tolerance in most but not all recipients, we observed an accelerated and early graft rejection of all donor allografts that lacked PDL1 on the endothelium. Moreover, PDL1-deficient endothelium hearts had significant higher frequency of IFN-γ-producing alloreactive cells as well as higher frequency of CD8(+) effector T cells. These findings demonstrate that PDL1 expression mainly on donor endothelium is functionally important in a fully allogeneic mismatched model for the induction of cardiac allograft tolerance.

Paradoxical functions of B7: CD28 costimulation in a MHC class II-mismatched cardiac transplant model.

Blockade of the B7: CD28 costimulatory pathway has emerged as a promising therapy to prevent allograft rejection. However, this pathway has also been demonstrated to be important for the generation and maintenance of regulatory T cells. In this study, we investigated the role of the B7: CD28 pathway in the 'bm12 into B6' MHC class II-mismatched vascularized cardiac transplant model of chronic rejection. Allograft rejection was remarkably accelerated in B6 background B7DKO and CD28KO recipients compared with B6 wild-type (WT) recipients. Allograft rejection was associated with a significantly enhanced Th1/Th2 alloreactivity and marked reduction in the ratio of regulatory T cells to CD4(+) effector/memory cells. We noted that administration of anti-B7-1 and anti-B7-2 mAb prior to transplantation also accelerated allograft rejection. Furthermore, depleting CD25(+) cells in B6 WT recipients of bm12 hearts prior to transplant also precipitated rejection at a similar rate. Neither B7/CD28 deficiency nor CD25 depletion affected graft survival in single MHC class I-mismatched (bm1 into B6) recipients. This study highlights the paradoxical functions of B7: CD28 costimulation in a MHC class II-mismatched model, in which the B7: CD28 pathway is demonstrated to be important in preventing rejection through the generation and maintenance of Tregs.

Acute hemiplegia associated with cat-scratch disease.

Cat scratch disease (CSD) is an infectious illness caused by a Gram-negative rod named Bartonella henselae. Typical CSD is characterized by a small skin lesion at the site of a scratch or a bite, followed by regional lymphadenopathy, one to two weeks later. Atypical forms may present as ocular manifestations, neurological manifestations, hepatosplenic involvement and vertebral osteomyelitis. Among neurological complications, encephalopathy is by far the most common. Other neurological manifestations are very rare. We report a case of an 11-year-old boy, with a posterior cervical lymphadenopathy and fever. Cat scratch disease was diagnosed and treated after a positive "Whartin-Starry" stain on lymph node biopsy. Two weeks after treatment, the patient was readmitted presenting an acute episode of left hemiplegia. A brain MRI demonstrated a right subcortical fronto-parietal lesion with no contrast enhancement. Complete recovery was observed after corticosteroid treatment.

Acute hemiplegia associated with cat-scratch disease

Cat scratch disease (CSD) is an infectious illness caused by a Gram-negative rod named Bartonella henselae. Typical CSD is characterized by a small skin lesion at the site of a scratch or a bite, followed by regional lymphadenopathy, one to two weeks later. Atypical forms may present as ocular manifestations, neurological manifestations, hepatosplenic involvement and vertebral osteomyelitis. Among neurological complications, encephalopathy is by far the most common. Other neurological manifestations are very rare. We report a case of an 11-year-old boy, with a posterior cervical lymphadenopathy and fever. Cat scratch disease was diagnosed and treated after a positive Whartin-Starry stain on lymph node biopsy. Two weeks after treatment, the patient was readmitted presenting an acute episode of left hemiplegia. A brain MRI demonstrated a right subcortical fronto-parietal lesion with no contrast enhancement. Complete recovery was observed after corticosteroid treatment.